Scatterometer, a lithographic apparatus and a focus analysis method

ABSTRACT

To detect whether a substrate is in a focal plane of a scatterometer, a cross-sectional area of radiation above a certain intensity value is detected both in front of and behind a back focal plane of the optical system of the scatterometer. The detection positions in front of and behind the back focal plane should desirably be equidistant from the back focal plane along the path of the radiation redirected from the substrate so that a simple comparison may determine whether the substrate is in the focal plane of the scatterometer.

FIELD

The present invention relates to a method of inspection usable, forexample, in the manufacture of devices by a lithographic technique andto a method of manufacturing devices using a lithographic technique.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

To determine features of a substrate, such as its alignment, a beam istypically redirected off the surface of the substrate, for example at analignment target, and an image is created on a camera of the redirectedbeam. By comparing a property of the beam before and after it has beenredirected by the substrate, a property of the substrate may bedetermined. This can be done, for example, by comparing the redirectedbeam with data stored in a library of known measurements associated witha known substrate property.

SUMMARY

When detecting features of a pattern, the pattern should be in the focalplane of the optics. A method for determining whether a pattern on asubstrate is in focus is the so-called “knife edge” method described inU.S. patent application publication no. US 2006-0066855, which documentis hereby incorporated in its entirety by reference. However, thismethod may complicated and require complex parts.

It is desirable, for example, to provide a method and apparatus fordetecting whether the substrate is in focus.

According to an aspect of the invention, there is provided ascatterometer configured to measure a property of a substrate, theapparatus comprising:

a high numerical aperture lens configured to project radiation onto thesubstrate and to project radiation redirected from the substrate towardsa back focal plane of the high numerical aperture lens or towards aconjugate of a front focal plane of the high numerical aperture lens;

a first detector configured to detect a cross-sectional area of theredirected radiation having an intensity above a first value; and

a second detector configured to detect a cross-sectional area of theredirected radiation having an intensity above a second value,

wherein the first detector is arranged in front of the back focal plane,between the high numerical aperture lens and the back focal plane, andthe second detector is arranged behind the back focal plane, or thefirst detector is arranged in front of the conjugate of the front focalplane, between the high numerical aperture lens and the conjugate of thefront focal plane, and the second detector is arranged behind theconjugate of the front focal plane.

According to an aspect of the invention, there is provided alithographic apparatus comprising:

a substrate table configured to hold a substrate;

a system configured to transfer a pattern onto the substrate; and

a scatterometer configured to measure a property of a substrate, theapparatus comprising:

-   -   a high numerical aperture lens configured to project radiation        onto the substrate and to project radiation redirected from the        substrate towards a back focal plane of the high numerical        aperture lens or towards a conjugate of a front focal plane of        the high numerical aperture lens,    -   a first detector configured to detect a cross-sectional area of        the redirected radiation having an intensity above a first        value, and    -   a second detector configured to detect a cross-sectional area of        the redirected radiation having an intensity above a second        value,    -   wherein the first detector is arranged in front of the back        focal plane, between the high numerical aperture lens and the        back focal plane, and the second detector is arranged behind the        back focal plane, or the first detector is arranged in front of        the conjugate of the front focal plane, between the high        numerical aperture lens and the conjugate of the front focal        plane, and the second detector is arranged behind the conjugate        of the front focal plane.

According to a further aspect of the invention, there is provided afocus analysis method for detecting whether a substrate is in the focalplane of a lens, the method comprising:

-   -   projecting radiation through a high numerical aperture lens and        onto the substrate;    -   detecting a first cross-sectional area of radiation redirected        by the substrate and passing through the high numerical aperture        lens, having an intensity above a first value, the detecting the        first cross-sectional area of the redirection radiation        occurring between the high numerical aperture lens and a back        focal plane of the high numerical aperture lens or between the        high numerical aperture lens and a conjugate of a front focal        plane of the high numerical aperture lens; and    -   detecting a second cross-sectional area of the redirected        radiation having an intensity above a second value, the        detecting the second cross-sectional area of the redirected        radiation occurring, respectively to the first detector, behind        the back focal plane or behind the conjugate of the front focal        plane.

According to a further aspect of the invention there is provided adevice manufacturing method comprising the focus control methoddescribed above. The focus control method described above may beimplemented using a control system.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 a depicts a lithographic apparatus;

FIG. 1 b depicts a lithographic cell or cluster;

FIG. 2 depicts a scatterometer;

FIG. 3 depicts a further scatterometer and the general operatingprinciple of measuring an angle resolved spectrum in the pupil plane ofa high-NA lens;

FIGS. 4 a and 4 b depict arrangements according to an embodiment of theinvention;

FIG. 5 depicts an further arrangement according to an embodiment of theinvention;

FIGS. 6A and 6C depicts patterns of radiation detected on the detectorwhen the substrate is in and out of focus;

FIGS. 7A and 7B depict detectors according to an embodiment of theinvention; and

FIGS. 8 to 10 depict further detectors according to an embodiment of theinvention.

DETAILED DESCRIPTION

FIG. 1 a schematically depicts a lithographic apparatus. The apparatuscomprises:

-   -   an illumination system (illuminator) IL configured to condition        a radiation beam B (e.g. UV radiation or EUV radiation);    -   a support structure (e.g. a mask table) MT constructed to        support a patterning device (e.g. a mask) MA and connected to a        first positioner PM configured to accurately position the        patterning device in accordance with certain parameters;    -   a substrate table (e.g. a wafer table) WT constructed to hold a        substrate (e.g. a resist-coated wafer) W and connected to a        second positioner PW configured to accurately position the        substrate in accordance with certain parameters; and    -   a projection system (e.g. a refractive projection lens system)        PL configured to project a pattern imparted to the radiation        beam B by patterning device MA onto a target portion C (e.g.        comprising one or more dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure holds the patterning device in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support structure can use mechanical, vacuum, electrostatic or otherclamping techniques to hold the patterning device. The support structuremay be a frame or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem. Any use of the terms “reticle” or “mask” herein may beconsidered synonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam, which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more support structures). In such“multiple stage” machines the additional tables and/or supportstructures may be used in parallel, or preparatory steps may be carriedout on one or more tables and/or support structures while one or moreother tables and/or support structures are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g. water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection systemand the substrate during exposure.

Referring to FIG. 1 a, the illuminator IL receives a radiation beam froma radiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the support structure (e.g., mask table) MT, and ispatterned by the patterning device. Having traversed the patterningdevice MA, the radiation beam B passes through the projection system PL,which focuses the beam onto a target portion C of the substrate W. Withthe aid of the second positioner PW and position sensor IF (e.g. aninterferometric device, linear encoder or capacitive sensor), thesubstrate table WT can be moved accurately, e.g. so as to positiondifferent target portions C in the path of the radiation beam B.Similarly, the first positioner PM and another position sensor (which isnot explicitly depicted in FIG. 1 a) can be used to accurately positionthe patterning device MA with respect to the path of the radiation beamB, e.g. after mechanical retrieval from a mask library, or during ascan. In general, movement of the support structure MT may be realizedwith the aid of a long-stroke module (coarse positioning) and ashort-stroke module (fine positioning), which form part of the firstpositioner PM. Similarly, movement of the substrate table WT may berealized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the support structure MT may be connected to ashort-stroke actuator only, or may be fixed. Patterning device MA andsubstrate W may be aligned using patterning device alignment marks M1;M2 and substrate alignment marks P1, P2. Although the substratealignment marks as illustrated occupy dedicated target portions, theymay be located in spaces between target portions (these are known asscribe-lane alignment marks). Similarly, in situations in which morethan one die is provided on the patterning device MA, the patterningdevice alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the support structure MT and the substrate table WT arekept essentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the support structure MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the supportstructure MT may be determined by the (de-)magnification and imagereversal characteristics of the projection system PL. In scan mode, themaximum size of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the support structure MT is kept essentiallystationary holding a programmable patterning device, and the substratetable WT is moved or scanned while a pattern imparted to the radiationbeam is projected onto a target portion C. In this mode, generally apulsed radiation source is employed and the programmable patterningdevice is updated as required after each movement of the substrate tableWT or in between successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

As shown in FIG. 1 b, the lithographic apparatus LA (controlled by alithographic apparatus control unit LACU) forms part of a lithographiccell LC, also sometimes referred to as a lithocell or lithocluster,which also includes apparatus to perform one or more pre- andpost-exposure processes on a substrate. Conventionally these include oneor more spin coaters SC to deposit a resist layer, one or moredevelopers DE to develop exposed resist, one or more chill plates CH andone or more bake plates BK. A substrate handler, or robot, RO picks up asubstrate from input/output ports I/O1, I/O2, moves it between thedifferent process devices and delivers it to the loading bay LB of thelithographic apparatus. These devices, which are often collectivelyreferred to as the track, are under the control of a track control unitTCU which is itself controlled by the supervisory control system SCS,which also controls the lithographic apparatus via lithographicapparatus control unit LACU. Thus, the different apparatus may beoperated to maximize throughput and processing efficiency.

In order that the substrate that is exposed by the lithographicapparatus is exposed correctly and consistently for each layer ofresist, it is desirable to inspect an exposed substrate to measure oneor more properties such as whether changes in alignment, rotation, etc.,overlay error between subsequent layers, line thickness, criticaldimension (CD), etc. If an error or change is detected, an adjustmentmay be made to an exposure of one or more subsequent substrates,especially if the inspection can be done soon and fast enough thatanother substrate of the same batch is still to be exposed. Also, analready exposed substrate may be stripped and reworked—to improve yield-or discarded—thereby avoiding performing an exposure on a substrate thatis known to be faulty. In a case where only some target portions of asubstrate are faulty, a further exposure may be performed only on thosetarget portions which are good. Another possibility is to adapt asetting of a subsequent process step to compensate for the error, e.g.the time of a trim etch step can be adjusted to compensate forsubstrate-to-substrate CD variation resulting from the lithographicprocess step.

An inspection apparatus is used to determine one or more properties of asubstrate, and in particular, how one or more properties of differentsubstrates or different layers of the same substrate vary from layer tolayer and/or across a substrate. The inspection apparatus may beintegrated into the lithographic apparatus LA or the lithocell LC or maybe a stand-alone device. To enable most rapid measurements, it isdesirable that the inspection apparatus measure one or more propertiesin the exposed resist layer immediately after the exposure

The one or more properties of the surface of a substrate W may bedetermined using a sensor such as a scatterometer such as that depictedin FIG. 2. The scatterometer comprises a broadband (white light)radiation projector 2 which projects radiation onto a substrate W. Thereflected radiation is passed to a spectrometer detector 4, whichmeasures a spectrum 10 (i.e. a measurement of intensity as a function ofwavelength) of the specular reflected radiation. From this data, thestructure or profile giving rise to the detected spectrum may bereconstructed by a processing unit, e.g. by Rigorous Coupled WaveAnalysis and non-linear regression or by comparison with a library ofsimulated spectra as shown at the bottom of FIG. 2. In general, for thereconstruction, the general form of the structure is known and someparameters are assumed from knowledge of the process by which thestructure was made, leaving only a few parameters of the structure to bedetermined from the scatterometry data. Such a scatterometer may beconfigured as a normal-incidence scatterometer or an oblique-incidencescatterometer. A variant of scatterometry may also be used in which thereflection is measured at a range of angles of a single wavelength,rather than the reflection at a single angle of a range of wavelengths.

A scatterometer for measuring one or more properties of a substrate maymeasure, in the pupil plane 11 of a high numerical aperture lens, theproperties of an angle-resolved spectrum reflected from the substratesurface W at a plurality of angles and wavelengths as shown in FIG. 3.Such a scatterometer may comprise a radiation projector 2 configured toproject radiation onto the substrate W and a detector 18 configured todetect the reflected spectra. The pupil plane is the plane in which theradial position of radiation defines the angle of incidence and theangular position defines azimuth angle of the radiation. The detector 14is placed in the pupil plane of the high numerical aperture lens. Thenumerical aperture of the lens may be high and desirably is at least 0.9or at least 0.95. An immersion scatterometer may even have a lens with anumerical aperture over 1.

An angle-resolved scatterometer only measures the intensity of scatteredradiation. However, a scatterometer may allow several wavelengths to bemeasured simultaneously at a range of angles. The properties measured bythe scatterometer for different wavelengths and angles may be theintensity of transverse magnetic- and transverse electric-polarizedradiation and/or the phase difference between the transverse magnetic-and transverse electric-polarized radiation.

Using a broadband radiation source (i.e. one with a wide range ofradiation frequencies or wavelengths—and therefore of colors) ispossible, which gives a large etendue, allowing the mixing of multiplewavelengths. The plurality of wavelengths in the broadband desirablyeach has a bandwidth of δλ and a spacing of at least 2δλ (i.e. twice thewavelength bandwidth). Several “sources” of radiation may be differentportions of an extended radiation source which have been split using,e.g., fiber bundles. In this way, angle resolved scatter spectra may bemeasured at multiple wavelengths in parallel. A 3-D spectrum (wavelengthand two different angles) may be measured, which contains moreinformation than a 2-D spectrum. This allows more information to bemeasured which increases metrology process robustness. This is describedin more detail in U.S. patent application publication no. US2006-0066855, which document is hereby incorporated in its entirety byreference.

A scatterometer that may be used with an embodiment of the presentinvention is shown in FIG. 3. The radiation of the radiation projector 2is collimated using lens system 12 through interference filter 13 andpolarizer 17, reflected by partially reflective surface 16 and isfocused onto substrate W via a microscope objective lens 15. Thereflected radiation is then transmitted through partially reflectivesurface 16 into a CCD detector 18 in the back projected pupil plane 11in order to have the scatter spectrum detected. The pupil plane 11 is atthe focal length of the lens system 15. A detector and high aperturelens are placed at the pupil plane. The pupil plane may be re-imagedwith auxiliary optics since the pupil plane of a high-NA lens is usuallylocated inside the lens.

A reference beam is often used, for example, to measure the intensity ofthe incident radiation. To do this, when the radiation beam is incidenton the partially reflective surface 16 part of it is transmitted throughthe surface as a reference beam towards a reference mirror 14. Thereference beam is then projected onto a different part of the samedetector 18.

The pupil plane of the reflected radiation is imaged on the CCDdetector, which may have an integration time of, for example, 40milliseconds per frame. In this way, a two-dimensional angular scatterspectrum of the substrate target is imaged on the detector. The detectormay be, for example, an array of CCD or CMOS sensors.

One or more interference filters 13 are available to select a wavelengthof interest in the range of, say, 405-790 nm or even lower, such as200-300 nm. The interference filter(s) may be tunable rather thancomprising a set of different filters. A grating could be used insteadof or in addition to one or more interference filters.

The target on substrate W may be a grating which is printed such thatafter development, the bars are formed of solid resist lines. The barsmay alternatively be etched into the substrate. The target pattern ischosen to be sensitive to a parameter of interest, such as focus, dose,overlay, chromatic aberration in the lithographic projection apparatus,etc., such that variation in the relevant parameter will manifest asvariation in the printed target. For example, the target pattern may besensitive to chromatic aberration in the lithographic projectionapparatus, particularly the projection system PL, and illuminationsymmetry and the presence of such aberration will manifest itself in avariation in the printed target pattern. Accordingly, the scatterometrydata of the printed target pattern is used to reconstruct the targetpattern. The parameters of the target pattern, such as line width andshape, may be input to the reconstruction process, performed by aprocessing unit, from knowledge of the printing step and/or otherscatterometry processes.

FIG. 4 a depicts an arrangement according to an embodiment of theinvention in which radiation is projected through the high numericalaperture lens 15 and through a focusing lens 21. The radiation is thenprojected onto a first detector 30 and a second detector 31. Asdescribed below, each of the detectors detects an amount (orcross-sectional area) of radiation above a predetermined intensitylevel. Each of the detectors may comprise one or more photodiodes, CCDsor CMOS. In this embodiment, the detectors are at least partiallytransmissive such that radiation is transmitted through the detectorsand onto one or more further optical elements. The detectors aredesirably arranged equidistant along the optical path from the backfocal plane of the high numerical aperture lens 15 or equidistant from aconjugate of the substrate plane, shown as the dashed line in FIG. 4 a.Assuming no transmissive losses between the detectors, if the substrateis in focus the cross-sectional area of the radiation above apredetermined intensity level (the spot size) will be the same at bothdetectors, as shown in both columns in FIG. 6 a, and a simple comparatorcan be used to determine whether the substrate is in focus. However, ifthe substrate is out of focus by being too far from the high numericalaperture lens, the spot size will be greater in the first detector(shown in the left column in FIG. 6 b) than the second detector (shownin the right column in FIG. 6 b). Conversely, if the substrate is out offocus by being to close to the high numerical aperture lens, the spotsize in the second detector (shown in the right column in FIG. 6 c) willbe greater than that in the first detector (shown in the left column inFIG. 6 c).

A further arrangement according to an embodiment of the invention isshown in FIG. 4 b. In this embodiment, a partially transmissive mirror22 is placed in the path of the beam after the high numerical aperturelens 15. The partially transmissive mirror 22 deflects a portion of theradiation towards a focus branch which includes the focusing lens 21together with first detector 30 and second detector 31. In thisembodiment the first detector 30 and second detector 31 are placedeither side and desirably equidistant of a conjugate of the substrateplane (a conjugate of the front focal plane of the high numericalaperture lens). The second detector need therefore not be partiallytransmissive. As an alternative to the transmissive mirror, a beamsplitter could also be used.

The predetermined intensity levels measured on the first and seconddetectors may not be the same. For example, if there are transmissivelosses between the first and second detectors, the predeterminedintensity level above which radiation is measured may be greater for thefirst detector than the second detector. Some calibration may berequired to determine the desired predetermined intensity levels.

Although these examples have just a first detector 30 and a seconddetector 31, each of the first detector and second detector could bedivided into a plurality of sub-detectors as shown in FIGS. 7 a and 7.FIG. 7 a depicts the first detector divided into a plurality of firstsub-detectors, 32, 33, 34 and FIG. 7 b depicts the second detectordivided into a plurality of second sub-detectors 37, 38, 39. The focusarea is then given by:

(I₃₂+I₃₄+I₃₈−(I₃₃+I₃₇+I₃₉)

where I₃₂ is the amount of radiation above a first predeterminedintensity level incident on sub-detector 32, I₃₇ is the amount ofradiation about a second predetermined intensity level incident onsub-detector 37, etc.

Although FIGS. 7 a and 7 b depict the first and second detectors dividedinto sub-detectors along a horizontal direction, the detectors could bedivided into sub-detectors in any number of ways. For example, FIG. 8depicts a detector divided into sub-detectors 42, 43, 44 along avertical direction. FIG. 9 depicts a detector divided into sub-detectors51 to 59 in a grid arrangement and FIG. 10 depicts a detector dividedinto sub-detectors 62, 63, 64 in concentric circles.

FIG. 5 depicts a further arrangement of the detectors shown in FIG. 4.In this embodiment, mirrors are used to project the radiation onto thedetectors. A partially reflective mirror 35 allows part of the radiationto pass through and onto first detector 30 while the remaining radiationreflects towards a second mirror 36 which reflects at least part of theradiation onto the second detector 31. The second mirror 36 may beeither fully reflective or partially reflective and the second detector31 may be transmissive to allow the radiation to be projected ontofurther optics. Again, the detectors are desirably arranged equidistantalong the path of the radiation from the back focal plane of the highnumerical aperture lens 15 or equidistant from a conjugate of thesubstrate plane.

Although the detectors are desirably arranged equidistant along the pathof the radiation from the back focal plane of the high numericalaperture lens 15 or from a conjugate of the substrate plane, they neednot be. If they are not equidistant from the back focal plane or aconjugate of the substrate plane, a calculation, rather than a simplecomparison, may determine whether the relative spot sizes on thedetectors indicate that the substrate is in focus or out of focus.

This method can be used in conjunction with an other, conventional focusdetection method. For example, one or more different focus detectionmethods may occupy different optical branches.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g. having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A scatterometer configured to measure a property of a substrate, theapparatus comprising: a high numerical aperture lens configured toproject radiation onto the substrate and to project radiation redirectedfrom the substrate towards a back focal plane of the high numericalaperture lens or towards a conjugate of a front focal plane of the highnumerical aperture lens; a first detector configured to detect across-sectional area of the redirected radiation having an intensityabove a first value; and a second detector configured to detect across-sectional area of the redirected radiation having an intensityabove a second value, wherein the first detector is arranged in front ofthe back focal plane, between the high numerical aperture lens and theback focal plane, and the second detector is arranged behind the backfocal plane, or the first detector is arranged in front of the conjugateof the front focal plane, between the high numerical aperture lens andthe conjugate of the front focal plane, and the second detector isarranged behind the conjugate of the front focal plane.
 2. Thescatterometer of claim 2, further comprising an angle detectorconfigured to detect an angle resolved spectrum of the redirectedradiation.
 3. The scatterometer of claim 1, wherein the first detectorand the second detector are arranged equidistant from the back focalplane or the conjugate of the front focal plane along an optical path ofthe redirected radiation.
 4. The scatterometer of claim 1, furthercomprising a comparator configured to comparing the cross-sectional areaof the redirected radiation having an intensity above the first valuedetected by the first detector and the cross-sectional area of theredirected radiation having an intensity above the second value detectedby the second detector.
 5. The scatterometer of claim 1, furthercomprising a first reflector configured to reflect the redirectedradiation towards the first detector.
 6. The scatterometer of claim 5,wherein the first reflector comprises a partially reflective mirror. 7.The scatterometer of claim 1, further comprising a second reflectorconfigured to reflect the redirected radiation towards the seconddetector.
 8. The scatterometer of claim 7, wherein the second reflectorcomprises, a partially reflective mirror.
 9. The scatterometer of claim1, wherein the first detector comprises a plurality of firstsub-detectors.
 10. The scatterometer of claim 1, wherein the seconddetector comprises a plurality of second sub-detectors.
 11. Thescatterometer of claim 1, wherein the first value is the same as thesecond value.
 12. The scatterometer of claim 1, wherein the first valueis greater than the second value.
 13. A lithographic apparatuscomprising: a substrate table configured to hold a substrate; a systemconfigured to transfer a pattern onto the substrate; and a scatterometerconfigured to measure a property of a substrate, the apparatuscomprising: a high numerical aperture lens configured to projectradiation onto the substrate and to project radiation redirected fromthe substrate towards a back focal plane of the high numerical aperturelens or towards a conjugate of a front focal plane of the high numericalaperture lens, a first detector configured to detect a cross-sectionalarea of the redirected radiation having an intensity above a firstvalue, and a second detector configured to detect a cross-sectional areaof the redirected radiation having an intensity above a second value,wherein the first detector is arranged in front of the back focal plane,between the high numerical aperture lens and the back focal plane, andthe second detector is arranged behind the back focal plane, or thefirst detector is arranged in front of the conjugate of the front focalplane, between the high numerical aperture lens and the conjugate of thefront focal plane, and the second detector is arranged behind theconjugate of the front focal plane.
 14. A focus analysis method fordetecting whether a substrate is in the focal plane of a lens, themethod comprising: projecting radiation through a high numericalaperture lens and onto the substrate; detecting a first cross-sectionalarea of radiation redirected by the substrate and passing through thehigh numerical aperture lens, having an intensity above a first value,the detecting the first cross-sectional area of the redirectionradiation occurring between the high numerical aperture lens and a backfocal plane of the high numerical aperture lens or between the highnumerical aperture lens and a conjugate of a front focal plane of thehigh numerical aperture lens; and detecting a second cross-sectionalarea of the redirected radiation having an intensity above a secondvalue, the detecting the second cross-sectional area of the redirectedradiation occurring, respectively to the first detector, behind the backfocal plane or behind the conjugate of the front focal plane.
 15. Themethod of claim 14, further comprising comparing the firstcross-sectional area of the redirected radiation and the secondcross-sectional area of the redirected radiation.
 16. The method ofclaim 14, further comprising detecting angles of a spectrum ofredirected radiation.
 17. A device manufacturing method, comprising:projecting a patterned beam of radiation onto a substrate; and detectingwhether a substrate is in the focal plane of a lens, the detectingcomprising: projecting radiation through a high numerical aperture lensand onto the substrate, detecting a first cross-sectional area ofradiation redirected by the substrate and passing through the highnumerical aperture lens, having an intensity above a first value, thedetecting the first cross-sectional area of the redirection radiationoccurring between the high numerical aperture lens and a back focalplane of the high numerical aperture lens or between the high numericalaperture lens and a conjugate of a front focal plane of the highnumerical aperture lens, and detecting a second cross-sectional area ofthe redirected radiation having an intensity above a second value, thedetecting the second cross-sectional area of the redirected radiationoccurring, respectively to the first detector, behind the back focalplane or behind the conjugate of the front focal plane.
 18. The methodof claim 17, further comprising comparing the first cross-sectional areaof the redirected radiation and the second cross-sectional area of theredirected radiation.
 19. A control system configured to control alithographic apparatus, the control system embodying executableinstructions configured to carry out a focus analysis method fordetecting whether a substrate is in the focal plane of a lens, themethod comprising: projecting radiation through a high numericalaperture lens and onto the substrate; detecting a first cross-sectionalarea of radiation redirected by the substrate and passing through thehigh numerical aperture lens, having an intensity above a first value,the detecting the first cross-sectional area of the redirectionradiation occurring between the high numerical aperture lens and a backfocal plane of the high numerical aperture lens or between the highnumerical aperture lens and a conjugate of a front focal plane of thehigh numerical aperture lens; and detecting a second cross-sectionalarea of the redirected radiation having an intensity above a secondvalue, the detecting the second cross-sectional area of the redirectedradiation occurring, respectively to the first detector, behind the backfocal plane or behind the conjugate of the front focal plane.
 20. Thecontrol system of claim 19, wherein the method further comprisescomparing the first cross-sectional area of the redirected radiation andthe second cross-sectional area of the redirected radiation.